Geochemistry Implications of Runoff at the Livestock and Forage Centre of Excellence

The objectives of this research were to determine the potential volume losses of water and associated constituent ions contained in cattle feedlot runoff through the runoff ditches at the USask Livestock and Forage Center of Excellence and the fate of the major anions and cations typically found in cattle feedlot runoff as they transport through the subsurface soils. This was accomplished through the investigation of the transport of five major cations (Ca2+, Mg2+, K+, Na+, and NH4+) and three major anions (Cl-, HCO3-, and SO42-) in laboratory soil columns. The columns were packed with two different soils: a clay present in the pen floors, and a silty clay present in the drainage ditches. The columns were then equilibrated with groundwater from the site. A simulated feedlot runoff solution was then flushed through the soil columns, followed by a simulated rainwater solution to represent season impacts. The breakthrough curves obtained by both stages of the experiment were used to identify key processes affecting the transport of the species. During the simulated runoff stage, Ca2+ was replaced on the exchange sites by Na+, K+, and NH4+ for all the columns (both soils). Generally, ions with higher valency will exchange for those of lower valency, but the majority of the ions in the feedlot runoff solution have a lower valency, replacing those of higher valency, such as Ca2+, through mass action. Ca2+ was replaced first, followed by Mg2+ once the ionic strength of the solution was higher. All columns had HCO3- concentrations below input concentrations. This was believed to be due to the precipitation of carbonate minerals. The pen clay columns demonstrated SO42- concentrations much higher than input concentrations. This, along with the high Ca2+ concentrations in the column effluent suggested that gypsum (CaSO4) dissolution was occurring from the soil. The solubility of gypsum at 25°C is 2000-2500 mg/L (Lebedev and Kosorukov 2017) and the average effluent concentration of SO42- was approximately 1600 mg/L which appeared to confirm the source of sulphate in the effluent. Simulated rainwater was then placed on top of the columns to assess the permanency of the attenuation of ions of interest. In all columns the effluent contained excess K+ and Na+, indicating the previously attenuated ions came back into solution off the exchange complex. There was also excess HCO3- and Ca2+, suggesting the dissolution of calcite. In the ditch silty clay columns, the concentration of Mg2+ was below the input concentration, indicating Mg2+ was sorbed on the exchange complex, replacing Na+, K+, and NH4+. In the pen clay columns, the SO42- and Ca2+ concentrations in the effluent remained well above the input concentrations. This suggests the dissolution of gypsum continued during the rainwater flushing stage.

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